A technique that monitors whether airtightness of a crystal resonator is maintained by monitoring changes in a current which flows in a piezoelectric resonator is known.
Examples of the related art include Japanese Laid-open Patent Publication No. 2010-232974 and Japanese Laid-open Patent Publication No. 06-177645.
However, in the above related art, it is difficult to initiate an alarm and delay output stop timing before the crystal resonator comes to an output stop (for example, a clock stop). Such an output stop of the crystal resonator may occur all of a sudden due to an abnormality or the like.